1. Field of the Invention
The present invention relates to an autofocus apparatus and a camera that are used for a camera, mainly for a video camera and particularly for a TV camera. The present invention also relates to a lens barrel and a camera that are used chiefly for a lens interchangeable type video camera and particularly for a lens interchangeable type TV camera.
2. Related Background Art
There have hitherto been two kinds of autofocus apparatuses as an autofocus apparatus for a camera. One type is generally called a crest climbing type autofocus apparatus used for mainly a video camera. The other type is called an image deviation type autofocus apparatus employed for chiefly a still camera. The two types of autofocus apparatuses will hereinafter be described with reference to the drawings.
First of all, the crest climbing type autofocus apparatus will be explained with reference to FIGS. 1 and 2. FIG. 1 is a schematic block diagram illustrating the crest climbing type autofocus apparatus. FIG. 2 is an explanatory graph showing a relationship between a level of a high frequency component of a picture signal.
The crest climbing type autofocus apparatus includes, as illustrated in FIG. 1, a photographing optical system 90 constructed of a focus adjusting lens 901, a variable magnification lens 902, a correcting lens 903, a stop 904 and an image forming lens 905. This autofocus apparatus also includes an imaging element 91 for picking up an image formed by the photographing optical system 90 and converting it into an electric signal, a picture signal making device 92 for generating a picture signal corresponding to the electric signal transmitted from the imaging element 91, and a focusing estimated value creating device 93 for creating an item of data (a focusing estimated value) for focusing the image formed on the imaging element 91 on the basis of the picture signal transmitted from the picture signal making device 92. The crest climbing type autofocus apparatus further includes a motor 94 for moving the focus adjusting lens 901 in an optical-axis direction of the photographing optical system 90, and a motor driving device 95 for driving the motor 94 referring to the focusing estimated value obtained by the focusing estimated value creating device 93.
Next, an operation of the crest climbing type autofocus apparatus will be explained. To begin with, the photographing optical system 90 forms an image on the imaging element 91. This image is, after being converted into the electric signal by the imaging element 91, further converted into the picture signal by the picture signal making device 92. This picture signal can be regarded as being formed by synthesizing sine waves of a plurality of frequencies.
By the way, it is well known that the level of the high frequency component of the picture signal, as shown in FIG. 2, rises more steeply as a degree of sharpness of the image formed on the imaging element 91 increases, viz., as the focus adjusting lens move closer to a focusing point A. Then, it is also a generally known fact that when the image on the imaging element 91 is focused on, the high frequency component level of the picture signal reaches its peak. Further, a crest of this level has a tendency of becoming steeper with a higher frequency. While on the other hand, an image forming performance of the lens and an S/N ratio of the picture signal worsen as the frequency gets higher.
Such being the case, the focusing estimated value creating device 93 selects a proper high frequency component from the picture signal obtained by the picture signal making device 92 in consideration of the S/N ratio of the picture signal as well as of the image forming performance of the photographing optical system 90, and creates a focusing estimated value by monitoring the level of this frequency component at an adequate sampling interval. For example, when the level of the selected frequency component is rising, it is estimated that the focus adjusting lens 901 is moving in such a direction as to approach a focusing point. Contrastingly, when the level of the selected frequency component is lowering, it is estimated that the focus adjusting lens 901 is moving in such a direction as to get away from the focusing point. Then, when the level of the selected frequency component exists within a predetermined range (shown by e.g., ΔV in FIG. 2), it is estimated that the image on the imaging element 91 is focused on.
Next, the motor driving device 95 refers to the focusing estimated values sequentially sent from the focusing estimated value creating device 93, and drives the motor 94 to move the focus adjusting lens 901 to such a position as to make a judgement of being focused on. Thus, in the crest climbing type autofocus apparatus, the focus adjusting lens 901 moves as if climbing the crest of the high frequency. This is the reason why the above autofocus apparatus is called the crest climbing type.
The above crest climbing type autofocus apparatus estimates the focusing by use of the picture signal and is therefore capable of highly accurately focusing the image on the imaging element 91 upon the corresponding object. Further, the image on the imaging element 91 is focused on without providing an element for an exclusive use of measuring a distance, and hence there must be an advantage costwise. Therefore, the apparatus is used as the autofocus apparatus mainly for the video camera.
Next, the image deviation type autofocus apparatus will be explained with reference to the drawings. FIGS. 3, 5 and 7 are explanatory diagrams each showing what the principle of the image deviation type autofocus apparatus is. Herein, FIG. 3 illustrates a light path when in a focused state. FIGS. 5 and 7 show the light paths when in a defocused state. Further, throughout the drawings, the numeral 96 designates an image forming optical system for forming the light beams into a conjugate image. The numeral 97 represents a predetermined focal surface of the image forming optical system 96, and 98a, 98b denote image re-forming lenses, disposed in positions substantially symmetric with respect to the optical axis of the image forming optical system 96, for re-forming some of the light beams (image) into images, which beams have been image-formed by the image forming optical system 96. Designated further by 99a, 99b are line sensors disposed in positions substantially symmetric with respect to the optical axis of the image forming optical system 96 and on predetermined focal surfaces of the image re-forming lenses 98a, 98b. 
As illustrated in FIG. 3, when the light beams passing through the image forming optical system are focused on the predetermined focal surface 97, some of the light beams are again image-formed on the line sensors 99a, 99b through the image re-forming lenses 98a, 98b. Accordingly, when focused on the predetermined focal surface 97, images E1, E2 picked up by the two line sensors 99a, 99b are, as illustrated in FIG. 4, formed in substantially coincident positions of the line sensors. On the other hand, as shown in FIG. 5, when the light beams passing through the image forming optical system are focused on anterior to the predetermined focal surface 97 (which is a so-called rear focus state), there must be a deviation between the images E1, E2 picked up by the two line sensors 99a, 99b as shown in FIG. 6. Further, as illustrated in FIG. 7, when the light beams passing through the image forming optical system are focused on posterior to the predetermined focal surface 97 (which is a so-called front focus state), there must be, as illustrated in FIG. 8, a deviation between the images picked up by the two line sensors 99a, 99b in a direction opposite to that in the rear focus state.
Hence, it is feasible to calculate a moving direction and a moving quantity of the focus adjusting lens, which are needed for focusing on the predetermined focal surface 97, by detecting a deviating direction and a deviation quantity between the images picked up by the line sensors 99a, 99b. As discussed above, in the image deviation type autofocus apparatus, the focusing is performed based on the deviating direction and the deviation quantity between the images picked up by the two line sensors 99a, 99b. This is the reason why the above autofocus apparatus is called the image deviation type.
The above-described image deviation type autofocus apparatus directly calculates the required-for-focusing moving direction and moving quantity of the focus adjusting lens on the basis of the deviating direction and the deviation quantity between the images picked up by the two line sensors 99a, 99b, and is therefore capable of focusing quickly. The image deviation type autofocus apparatus is therefore used mainly for the still camera.
Principally, the crest climbing type autofocus apparatus has hitherto been used for the video camera. The video camera to which the conventional crest climbing type autofocus apparatus is applied, will hereinafter be explained with reference to the drawings. FIG. 9 is a schematic block diagram showing the video camera to which the prior art crest climbing type autofocus apparatus is applied. FIG. 2 is an explanatory graph showing how a level of a high frequency of the picture signal relates to a position of the focus adjusting lens.
The video camera to which the conventional crest climbing type autofocus apparatus is applied includes, as illustrated in FIG. 9, a lens barrel 8 and a camera body 9 to which the lens barrel 8 is attached.
The lens barrel 8 is so constructed as to be attachable to the camera body 9, and a user is able to interchange the lens barrel 8 attached to the camera body 9 according to applications thereof. Further, the lens barrel 8 has a photographing optical system 120 constructed of a focus adjusting lens 801, a variable magnification lens 802, a correcting lens 803, a stop 804 and an image forming lens 805. The lens barrel 8 also has a motor 122 for moving the focus adjusting lens 801 in the optical-axis direction of the photographing optical system 120, and a motor driving circuit 123 for driving the motor 122 based on a motor driving signal transmitted via a connector 125.
The camera body 9 includes an imaging element 91 for picking up the image obtained by the photographing optical system 120 and converting it into an electric signal, a picture signal making device 92 for generating a picture signal corresponding to the electric signal transmitted from the imaging element 91, and a focusing estimated value creating device 93 for creating the data (a focusing estimated value) for focusing the image on the imaging element 91 on the basis of the picture signal from the picture signal making device 92. The camera body 9 also includes an AF processing circuit 104 for generating a motor driving signal with reference to the focusing estimated value obtained by the focusing estimated value creating device 93, and a connector 105 through which the motor driving signal generated by the AF processing circuit 104 is transmitted to the motor driving circuit 123 of the lens barrel 8. Note that the connector 105 is so constructed as to be electrically connected to a connector 125 of the lens barrel when the lens barrel 8 is attached to the camera body 9.
Next, an autofocusing operation of the video camera shown in FIG. 9 will be explained. At the first onset, the photographing optical system 120 forms the image on the imaging element 91. This image is converted into the electric signal by the imaging element 91 and thereafter converted further into the picture signal by the picture signal making device 92. This picture signal can be regarded as being formed by synthesizing sine waves of a plurality of frequencies.
By the way, it is well known that the level of the high frequency component of the picture signal, as shown in FIG. 2, rises more steeply as a degree of sharpness of the image formed on the imaging element 91 increases, viz., as the focus adjusting lens move closer to a focusing point A. Then, it is also a generally known fact that when the image on the imaging element 91 is focused on, the high frequency component level of the picture signal reaches its peak. Further, a crest of this level has a tendency of becoming steeper with a higher frequency. While on the other hand, an image forming performance of the lens and an S/N ratio of the picture signal worsen as the frequency gets higher.
Such being the case, the focusing estimated value creating device 93 selects a proper high frequency component from the picture signal obtained by the picture signal making device 92 in consideration of the S/N ratio of the picture signal as well as of the image forming performance of the photographing optical system 120, and creates a focusing estimated value by monitoring the level of this frequency component at an adequate sampling interval. For example, when the level of the selected frequency component is rising, it is estimated that the focus adjusting lens 801 is moving in such a direction as to approach a focusing point. Contrastingly, when the level of the selected frequency component is lowering, it is estimated that the focus adjusting lens 801 is moving in such a direction as to get away from the focusing point. Then, when the level of the selected frequency component exists within a predetermined range (shown by e.g., ΔV in FIG. 2), it is estimated that the image on the imaging element 91 is focused on.
Next, the AF processing circuit 104 refers to the focusing estimated values sequentially sent from the focusing estimated value creating device 93, and generates a motor driving signal to move the focus adjusting lens 801 to such a position as to make a judgement of being focused on. The motor driving circuit 123 drives the motor 122 based on the motor driving signal generated by the AF processing circuit 104.
Thus, in the crest climbing type autofocus apparatus, the focus adjusting lens 801 moves as if climbing the crest of the high frequency. This is the reason why the above autofocus apparatus is called the crest climbing type.
The video camera to which the above crest climbing type autofocus apparatus is applied, estimates the focusing by use of the picture signal and is therefore capable of well accurately focusing on the object corresponding to the image on the imaging element 91. Further, this video camera is advantageous costwise because of focusing the image on the imaging element 91 without providing an element for an exclusive use of measuring a distance.
In the above-described crest climbing type autofocus apparatus, the focus adjusting lens 901 is moved to such a position as to make the judgement of being focused while referring to the focusing estimated values sequentially transmitted from the focusing estimated value creating device 93 at a predetermined sampling interval. This might lead to a problem, in which it takes much time to attain the focusing. Especially when the focus adjusting lens 901 is positioned far from the focusing point (which is a so-called largely defocused state), as illustrated in FIG. 2, a variation in the high frequency component level of the picture signal might be small enough to be hidden by noises, etc., or there must be a high possibility of not indicating a peak of the level thereof. Consequently, the focusing estimated value creating device 93 becomes harder to make the estimation. Therefore, the time till the focusing is attained might extremely elongate in some cases.
The problem of requiring a good deal of time for focusing might bring about a possibility in which the photographer would miss an instantaneous photographing chance on the occasion of reporting news and so forth when, e.g., the above crest climbing type autofocus apparatus is used for the TV camera.
In this respect, the above image deviation type autofocus apparatus is, as stated earlier, capable of focusing quickly. When the image deviation type autofocus apparatus is employed for the video camera, however, it is required that the light beams be diverged from the photographing optical system for forming the image on the imaging element, and the image be formed in a position different from the imaging element surface. Therefore, whether or not the image on the imaging element be focused on is estimated by making use of the image formed in the position different from the imaging element surface. Hence, this leads to a problem of being incapable of focusing with a high accuracy. The high accuracy (a deviation on the image surface in the optical-axis direction of the photographing optical system is within approximately several μm), is required of particularly the autofocus apparatus for the TV camera. Meeting this accuracy in the above image deviation type autofocus apparatus must involve a remarkable increase in costs.
Further, in the video camera to which the above prior art crest climbing type autofocus apparatus is applied, the focus adjusting lens 801 is moved to such a position as to make the judgement of being focused while referring to the focusing estimated values sequentially transmitted from the focusing estimated value creating device 93 at the predetermined sampling interval. Therefore, the problem is that the focusing might need much time. In particular, when the focus adjusting lens 801 is positioned far from the focusing point (which is the so-called largely defocused state), as shown in FIG. 2, the level of the high frequency component of the picture signal remains almost unchanged even by moving the focus adjusting lens 801. Hence, the focusing estimated value creating device 93 is hard to make the estimation in terms of considering the noises, etc. contained in the high frequency components. Consequently, the time needed till the focusing is attained might extremely elongate in some cases. The problem of requiring a good deal of time till the focusing is done might further conduce to the possibility, wherein the photographer would miss the instantaneous photographing chance on the occasion of reporting news when, e.g., the above-described crest climbing type autofocus apparatus is used for the TV camera.
Moreover, in the video camera to which the above prior art crest climbing type autofocus apparatus is applied, as illustrated in FIG. 9, the camera body incorporates the focusing estimated value creating device and the AF processing device. Therefore, in the video camera, both of the camera body corresponding the autofocusing and the lens barrel corresponding to the relevant camera body, are needed for actualizing the autofocus function. Particularly the camera body of the TV camera employed by a professional cameraman is, however, very expensive and therefore causes such a problem that the user is forced to expend much in order to actualize the autofocus function in that TV camera.
Furthermore, when replacing the lens barrel attached to the video camera body, a so-called tracking adjustment is required to be performed, wherein a predetermined focal surface of the lens barrel is adjusted to the imaging element surface of the video camera body by moving the image forming lens of the lens barrel in the optical-axis direction of the lens barrel. Upon carrying out the tracking adjustment, there must be produced a deviation in an optical positional relationship between the imaging element surface of the video camera body and the predetermined focal surface 97 for estimating the focusing, resulting in a decline in terms of the focusing accuracy.